A battery cell serves as a power supplying source for an electrical tool, and its continuous operational capability is usually related to its electrical energy capacity. Increasing the electrical energy capacity of the battery cell can improve its continuous operational duration, which increases the weight of the battery cell and is contradictory to a purpose to obtain flexible operation of the battery cell. Besides, no matter how the electrical energy capacity is increased, indefinite continuous operation is unlikely to be achieved. Hence, currently a method for solving the problem of insufficient power quantity is to improve a charging efficiency of the battery cell, namely to employ a fast-charging manner to supplement sufficient power quantity for the battery cell in a time period that a user can wait.
A general charging method is to load a constant current on the battery load. In a beginning phase, the battery cell, as having a low power quantity, can allow electrical energy to be supplied quickly, and it is characterized in that the current value passing through the battery cell is constant in this phase. The electrical energy accepting capability of the battery cell falls after a period of time, and the battery cell is characterized in that a voltage at both ends thereof tends to be stable. The above charging manner is called a constant current-constant voltage (CC-CV) manner, wherein in the constant current (CC) phase the current is approximately stable, the charging quantity of the battery cell is in direct proportion to the current, and the charging speed is fast in this phase; when the charging enters the constant voltage (CV) phase, the charging current decreases along with the increase of the electrical charge of the battery cell; in the constant voltage phase, the charging speed is slow. Usually when the current decreases to a certain degree (for example, 0.05 C), the battery cell is believed to be fully charged, and the current is considered as a condition for stopping the charging and the charger automatically stops the charging.
Take charging of a SANYO UR18650RX lithium battery cell with 2.0 A as an example. Referring to Table 1 and FIG. 1, its constant current phase is about 45 minutes, its constant voltage phase is about 16 minutes, the constant voltage phase accounts for about 26% of the total charging time, that is, the constant voltage phase of the low-efficiency charging accounts for one quarter of the total charging time, which obviously does not meet the fast-charge requirements.
TABLE 1ConstantConstantTotalPercentagecurrent phasevoltage phasecharging timeof CV phase2.0 A45166126%4.0 A26154137%6.0 A16143048%
According to ideas of those having ordinary skill in the art, the charging speed is generally quickened in a manner of increasing the current, and this manner is also a current fast-charge manner, for example, as shown in Table 1 and FIG. 2 and FIG. 3, in the case that the charging current is increased to 4.0 A and 6.0 A respectively to charge the previous SANYO UR18650RX lithium battery cell, it is found that as the charging current increases, the constant voltage phase accounts for a larger proportion of the total charging time. This fast-charge method is not worthwhile.
Generally speaking, when a battery cell leaves the factory, the battery cell manufacturer will indicate a recommended charging current value and battery cell capacity for the charging in the CC-CV manner, and correspondingly provide a charge cut-off current value and a corresponding cut-off voltage value. A battery cell assembly is composed of battery cells, and it has a rated charging voltage value and a rated charging current value which are according to the cut-off current value, cut-off voltage value and the connection of the battery cells.
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.